Liquid crystal display

ABSTRACT

There is provided a liquid crystal display device in which the wiring resistivity of signal lines is reduced. The liquid crystal display device includes substrates disposed in opposition to each other with a liquid crystal interposed therebetween, a thin film transistor to be driven by a scanning signal supplied from a gate signal line, and a pixel electrode to be supplied with a video signal from a drain signal line via the thin film transistor, the thin film transistor and the pixel being provided in each pixel area on a liquid-crystal-side surface of one of the substrates. The gate signal line is made of a multi-layered structure including at least an ITO film formed on the liquid-crystal-side surface and a Mo layer formed to overlie the ITO film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, andmore particularly, to an active matrix type of liquid crystal displaydevice.

2. Background Art

An active matrix type of liquid crystal display device includes pixelareas provided on a liquid-crystal-side surface of either one ofsubstrates disposed in opposition to each other with a liquid crystalinterposed therebetween, each of the pixel areas being an areasurrounded by gate signal lines disposed to be extended in the xdirection and to be juxtaposed in the y direction and drain signal linesdisposed to be extended in the y direction and to be juxtaposed in the xdirection.

Each of the pixel areas includes a switching element to be driven by agate signal from either one of the gate signal lines, and a pixelelectrode to be supplied with a video signal from a drain signal linevia the switching element.

Two types of liquid crystal display devices are known. One is aso-called vertical electric field type of liquid crystal display devicein which a counter electrode common to each pixel area is formed on theliquid-crystal-side surface of one of substrates so that the opticaltransmissivity of its liquid crystal is controlled by an electric fieldgenerated between the counter electrode and a pixel electrodeapproximately perpendicularly to the substrates. The other is aso-called in-plane-switching type of liquid crystal display device inwhich a pixel electrode and an adjacent counter electrode are formed ineach pixel area on a substrate on which pixel electrodes are formed, sothat the optical transmissivity of its liquid crystal is controlled byan electric field generated between the pixel electrode and the counterelectrode approximately in parallel with the substrate.

However, it has been demanded that the gate signal lines or the drainsignal lines of such a liquid crystal display device have smaller wiringresistivity to cope with a recent increase in panel size.

In other words, by reducing the wiring resistivity of these signallines, it is possible to restrain the delay of signals, whereby it ispossible to achieve far larger panel sizes.

However, even if the wiring resistivity of these signal lines can bereduced, it is necessary to avoid an increase in the number ofmanufacturing processes because yield factor decreases.

In addition, in the case where a conductive layer, a semiconductor layerand an insulating layer are stacked in a predetermined pattern on aliquid-crystal-surface side of a substrate, the resultant steep stepsare required to be made as smooth as possible in terms of an improvementin the yield factor. This is because a film undergoes climb-over damagesin portions where the steps are present.

SUMMARY OF THE INVENTION

The invention has been made in view of the above-described problems, andone object of the invention is to provides a liquid crystal displaydevice in which the wiring resistivity of signal lines is small.

Another object of the invention is to provide a liquid crystal displaydevice in which few steep steps are present on a liquid-crystal-sidesurface of a substrate.

Another object of the invention is to provide a manufacturing method fora liquid crystal display device in which the number of manufacturingprocesses is made small.

Representative aspects of the invention disclosed in the presentapplication will be described below in brief.

A liquid crystal display device according to the invention includes:substrates disposed in opposition to each other with a liquid crystalinterposed therebetween; a thin film transistor to be driven by ascanning signal supplied from a gate signal line; and a pixel electrodeto be supplied with a video signal from a drain signal line via the thinfilm transistor, the thin film transistor and the pixel being providedin each pixel area on a liquid-crystal-side surface of one of thesubstrates. The gate signal line is made of a multi-layered structureincluding, for example, an ITO film formed on the liquid-crystal-sidesurface, and a Mo layer formed to overlie the ITO film.

In the liquid crystal display device constructed in this manner, thewiring resistivity of the gate signal lines is made small by using Mo orthe like which has a small resistivity.

In this case, the reason why the ITO film or the like is interposedbetween the Mo layer and the substrate is that if the gate signal lineis made of a single layer of Mo or the like, the adhesion of the gatesignal line to the substrate becomes insufficient.

In the case where the gate signal line made of such multi-layeredstructure is formed by selective etching, the side walls of the gatesignal line are respectively formed to have tapered surfaces whichbecome gradually more open toward the substrate, whereby it is possibleto decrease steep steps.

A manufacturing method for a liquid crystal display device according tothe invention includes the steps of: forming, on a substrate, gatesignal lines each made of a stacked structure in which a transparentconductive film and a metal layer are stacked in that order; forming aninsulating film to cover the gate signal lines; forming, on theinsulating film, a stacked structure in which a semiconductor layer, ahigh-concentration layer and a conductive layer are stacked in thatorder; performing selective etching of the conductive layer and thehigh-concentration layer by using a resist reflow method, to form drainelectrodes and source electrodes for thin film transistors as well asdrain signal lines and to perform selective etching of the semiconductorlayer; forming pixel electrodes each of which is in part directlysuperposed on the source electrode of the corresponding one of the thinfilm transistors; and forming a protective film and opening, in theprotective film, apertures for exposing the respective pixel electrodes.

In the manufacturing method for the liquid crystal display deviceconstructed in this manner, although the formation of the semiconductorlayer and the formation of the drain electrode and the source electrodehave heretofore been performed with individual photo-processes, suchindividual photo-process can be replaced with one photo-process by usingthe resist reflow method, whereby it is possible to reduce the entiremanufacturing process.

At the same time that holes are formed in the protective film, holes atgate terminal parts and drain terminal parts are formed, whereby it ispossible to reduce the entire manufacturing process.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more readily appreciated and understood fromthe following detailed description of preferred embodiments of theinvention when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a plan view showing one embodiment of a pixel of a liquidcrystal display device according to the invention;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 1;

FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 1;

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 1;

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 1;

FIGS. 7A to 7C are process diagrams showing one embodiment of amanufacturing method for the gate signal lines GL shown in FIG. 1;

FIGS. 8A to 8F are process diagrams showing one embodiment of amanufacturing method for the thin film transistor shown in FIG. 1;

FIGS. 9A to 9H are process diagrams showing one embodiment of amanufacturing method for the liquid crystal display device shown in FIG.1;

FIG. 10 is a table showing one embodiment of a manufacturing method forthe liquid crystal display device shown in FIG. 1;

FIG. 11 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a thin film transistor;

FIG. 12 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a drain signal line and the vicinity thereof;

FIG. 13 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a gate terminal part;

FIG. 14 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a drain terminal part;

FIGS. 15A to 15E are views showing another embodiment of the liquidcrystal display device according to the invention, and are processdiagrams showing one embodiment of a manufacturing method for a thinfilm transistor;

FIGS. 16A to 16G are process diagrams showing one embodiment of amanufacturing method for the liquid crystal display device according tothe invention;

FIG. 17 is a table showing one embodiment of a manufacturing method forthe liquid crystal display device according to the invention;

FIG. 18 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a thin film transistor;

FIG. 19 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a drain signal line and the vicinity thereof;

FIG. 20 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a gate terminal part;

FIG. 21 is a view showing another embodiment of the liquid crystaldisplay device according to the invention, and showing a cross sectionof a drain terminal part;

FIGS. 22A to 22G are views showing another embodiment of the liquidcrystal display device according to the invention, and are processdiagrams showing one embodiment of a manufacturing method for a thinfilm transistor;

FIGS. 23A to 23G are process diagrams showing one embodiment of amanufacturing method for the liquid crystal display device according tothe invention;

FIG. 24 is a table showing one embodiment of a manufacturing method forthe liquid crystal display device according to the invention;

FIG. 25 is a plan view showing one embodiment of a pixel of the liquidcrystal display device according to the invention;

FIG. 26 is a cross-sectional view taken along line 26-26 of FIG. 25;

FIG. 27 is a cross-sectional view taken along line 27-27 of FIG. 25;

FIG. 28 is a plan view showing one embodiment of a pixel of the liquidcrystal display device according to the invention;

FIG. 29 is a cross-sectional view taken along line 29-29 of FIG. 28;

FIG. 30 is a cross-sectional view taken along line 30-30 of FIG. 28;

FIG. 31 is a plan view showing one embodiment of a pixel of the liquidcrystal display device according to the invention;

FIG. 32 is a cross-sectional view taken along line 32-32 of FIG. 31;

FIG. 33 is a cross-sectional view taken along line 33-33 of FIG. 31;

FIG. 34 is a cross-sectional view taken along line 34-34 of FIG. 31;

FIG. 35 is a cross-sectional view showing one embodiment of a gateterminal part of the liquid crystal display device shown in FIG. 25;

FIG. 36 is a cross-sectional view showing one embodiment of a drainterminal part of the liquid crystal display device shown in FIG. 25;

FIGS. 37A to 37F are process diagrams showing one embodiment of amanufacturing method for a thin film transistor of the liquid crystaldisplay device shown in FIG. 25;

FIGS. 38A to 38E are process diagrams showing one embodiment of theliquid crystal display device shown in FIG. 25;

FIG. 39 is a table showing one embodiment of the liquid crystal displaydevice shown in FIG. 25;

FIG. 40 is a plan view showing one embodiment of a pixel of the liquidcrystal display device according to the invention;

FIG. 41 is a cross-sectional view taken along line 41-41 of FIG. 40;

FIG. 42 is a cross-sectional view taken along line 42-42 of FIG. 40;

FIG. 43 is an explanatory view showing a half-exposure method;

FIG. 44 is a cross-sectional view taken along line 44-44 of FIG. 40;

FIG. 45 is a cross-sectional view taken along line 45-45 of FIG. 40;

FIGS. 46A to 46D are process diagrams showing one embodiment of amanufacturing method for the thin film transistor shown in FIG. 40;

FIGS. 47E to 47H are process diagrams following FIG. 46D, showing oneembodiment of a manufacturing method for the thin film transistor shownin FIG. 40;

FIG. 48I are a process diagram following FIG. 47H, showing oneembodiment of a manufacturing method for the thin film transistor shownin FIG. 40;

FIG. 49 is a plan view showing one embodiment of a pixel of the liquidcrystal display device according to the invention;

FIG. 50 is a cross-sectional view taken along line 50-50 of FIG. 49;

FIG. 51 is a cross-sectional view taken along line 51-51 of FIG. 49;

FIG. 52 is a cross-sectional view showing the drain terminal part shownin FIG. 49;

FIGS. 53A to 53D are process diagrams showing one embodiment of amanufacturing method for the thin film transistor shown in FIG. 49; and

FIG. 54 is a view showing one embodiment of the equivalent circuit ofthe liquid crystal display device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the liquid crystal display device according to theinvention will be described below with reference to the accompanyingdrawings.

Embodiment 1.

<<Equivalent Circuit>>

FIG. 54 is an equivalent circuit diagram showing one embodiment of theliquid crystal display device according to the invention. FIG. 54 is acircuit diagram which is depicted to correspond to the actualgeometrical layout of the liquid crystal display device.

In FIG. 54, there is shown a transparent substrate SUB1. Thistransparent substrate SUB1 is disposed to be opposed to anothertransparent substrate SUB2 with a liquid crystal interposedtherebetween.

Gate signal lines GL and drain signal lines DL are formed on aliquid-crystal-side surface of the transparent substrate SUB1. The gatesignal lines GL are disposed to be extended in the x direction and to bejuxtaposed in the y direction as viewed in FIG. 54, while the drainsignal lines DL are insulated from the gate signal lines GL and aredisposed to be extended in the y direction and to be juxtaposed in the xdirection as viewed in FIG. 54. Rectangular areas each of which issurrounded by adjacent ones of the gate signal lines GL and adjacentones of the drain signal lines DL constitute pixel areas, respectively,and a display part AR is formed by an aggregation of these pixel areas.

A thin film transistor TFT and a pixel electrode PX are formed in eachof the pixel areas. The thin film transistor TFT is driven by the supplyof a scanning signal (voltage) from one of the adjacent gate signallines GL, and a video signal (voltage) is supplied to the pixelelectrode PX from one of the adjacent drain signal lines DL via the thinfilm transistor TFT.

A capacitance element Cadd is formed between the pixel electrode PX andthe other of the adjacent gate signal lines GL so that when the thinfilm transistor is turned off, a video signal supplied to the pixelelectrode PX is stored in the capacitance element Cadd for a long time.

The pixel electrode PX in each of the pixel areas is arranged togenerate an electric field between the pixel electrode PX and a counterelectrode CT (not shown) formed in common to each of the pixel areas ona liquid-crystal-side surface of the other transparent substrate SUB2which is disposed to be opposed to the transparent substrate SUB1 withthe liquid crystal interposed therebetween. The optical transmissivityof the liquid crystal between each of the pixel electrodes PX and thecounter electrode CT is controlled by the electric field.

One end of each of the gate signal lines GL is formed to be extended toone side (in FIG. 54, the left-hand side) of the transparent substrateSUB1, and the extended portion (which will be hereinafter called a gateterminal part GTM) is connected to a bump of a vertical scanning circuit(semiconductor integrated circuit) V mounted on the transparentsubstrate SUB1.

In addition, one end of each of the drain signal lines DL is formed tobe extended to one side (in FIG. 54, the top side) of the transparentsubstrate SUB1, and the extended portion (which will be hereinaftercalled a drain terminal part DTM) is connected to a video signal drivercircuit (semiconductor integrated circuit) He mounted on the transparentsubstrate SUB1.

The transparent substrate SUB2 is disposed to be opposed to the area(display part AR) of the transparent substrate SUB1 that avoids an areain which the vertical scanning circuit V and the video signal drivercircuit He are mounted.

The transparent substrate SUB2 is secured to the transparent substrateSUB1 by a sealing material SL formed in the periphery of the transparentsubstrate SUB2, and this sealing material SL also has the function ofsealing the liquid crystal between the transparent substrates SUB1 andSUB2.

<<Construction of Pixel>>

FIG. 1 shows the construction of one pixel (corresponding to the portionshown in a dotted frame A of FIG. 54) on the liquid-crystal-side surfaceof the transparent substrate SUB1, as well as the constructions ofterminal parts for supplying a scanning signal and a video signal to thepixel. FIG. 2 shows a cross-sectional view taken along line 2-2 of FIG.1, FIG. 3 shows a cross-sectional view taken along line 3-3 of FIG. 1,FIG. 4 shows a cross-sectional view taken along line 4-4 of FIG. 1, FIG.5 shows a cross-sectional view taken along line 5-5 of FIG. 1, and FIG.6 shows a cross-sectional view taken along line 6-6 of FIG. 1.

The gate signal lines GL disposed to be extended in the x direction andto be juxtaposed in the y direction as viewed in FIG. 1 are formed onthe liquid-crystal-side surface of the transparent substrate SUB1. Eachof these gate signal lines GL is made of a two-layer structure whichincludes an ITO (Indium-tin-Oxide) film as its lower layer and amolybdenum (MO) film as its upper layer.

One end (in FIG. 1, the left-hand side) of each of the gate signal linesGL is formed to be extended outwardly from the display part AR, and theextended portion is formed as the gate terminal part GTM having acomparatively large area.

The gate signal lines GL and the drain signal lines DL which will bedescribed later surround the rectangular area, and this rectangular areais constructed as a pixel area.

A portion of each of the gate signal lines GL has an extended portionwhich projects into the pixel area, and this extended portion has thefunction of the gate electrode of the thin film transistor TFT whichwill be described later.

In addition, another portion of each of the gate signal lines GL has anextended portion which projects into another pixel area disposedadjacently to the pixel area in the -y direction, and this extendedportion has the function of one electrode of the capacitance elementCadd (the other electrode of which is the pixel electrode PX).

In the pixel area, light shield films SKD are formed to run adjacentlyto and in parallel with the respective drain signal lines DL. Theselight shield films SKD are formed in parallel with the gate signal linesGL, and have a two-layer structure which includes an ITO film as itslower layer and a Mo film as its upper layer.

These light shield films SKD, together with a black matrix BM formed onthe glass substrate SUB2, can highly reliably block light due to thedisorder of alignment of the liquid crystal in the periphery of thepixel electrode PX (particularly, along the sides of the pixel electrodePX that are parallel to the y direction in FIG. 1).

An insulating film GI (refer to FIGS. 2 and 6) is formed on theliquid-crystal-side surface of the transparent substrate SUB1 on whichthe gate signal lines GL and the light shield films SKD are formed inthe above-described manner. This insulating film GI has the function ofan interlayer insulating film between the gate signal lines GL and thedrain signal lines DL which will be described later, the function ofgate insulating films for the thin film transistors TFT which will bedescribed later, and the function of dielectric films for thecapacitance elements Cadd which will be described later.

A semiconductor layer AS made of, for example, amorphous Si (a-Si) isformed on the upper surface of the insulating film GI in such a manneras to traverse the extended portion of each of the gate signal lines GLthat projects into the pixel area.

This semiconductor layer AS constitutes the semiconductor layer of thethin film transistor TFT, and a drain electrode SD1 and a sourceelectrode SD2 are formed on the upper surface of the semiconductor layerAS, whereby a reversed staggered structure MIS transistor is formedwhich uses the extended portion of the gate signal line GL as its gateelectrode.

Incidentally, this semiconductor layer AS is formed not only in theregion in which the thin film transistor TFT is formed, but is formed tobe integrally extended into the region in which the drain signal line DLto be described later is formed. This construction is intended tostrengthen the interlayer insulation of the drain signal line DL fromthe gate signal line GL.

The drain electrode SD1 and the source electrode SD2 on thesemiconductor layer AS are formed at the same time as the drain signalline DL.

Specifically, the drain signal lines DL which are disposed to beextended in the y direction and to be juxtaposed in the x direction asviewed in FIG. 1 are formed on the upper surface of the insulating filmGI (the semiconductor layer AS underlies the drain signal lines DL), anda part of each of the drain signal lines DL is formed to be extended tothe upper surface of the semiconductor layer AS of the thin filmtransistor TFT and forms the drain electrode SD1.

Incidentally, one end (in FIG. 1, the top side) of each of the drainsignal lines DL is formed to be extended outwardly from the display partAR, and the extended portion is formed as the gate terminal part GTMhaving a comparatively large area.

The source electrode SD2 is formed to be spaced apart from the drainelectrode SD1 by a distance equivalent to the channel length of the thinfilm transistor TFT.

This source electrode SD2 is formed to be extended into the pixel areafrom above the semiconductor layer AS of the thin film transistor TFT,and this extended portion is formed as a part for connection to thepixel electrode PX which will be described later.

The drain signal lines DL, the drain electrodes SD1 and the sourceelectrodes SD2 are formed of, for example, molybdenum (Mo).

Incidentally, a high-concentration layer d₀ doped with an impurity isformed at the interface between the semiconductor layer AS and each ofthe source electrode SD2 and the drain electrode SD1 (refer to FIG. 2).This high-concentration layer d₀ functions as a contact layer of thethin film transistor TFT.

In addition, the high-concentration layer d₀ is formed at the interfacebetween the drain signal lines DL and the underlying semiconductor layerAS (refer to FIG. 4).

The pixel electrode PX made of, for example, ITO (Indium-Tin-Oxide) filmis formed on the central portion of the upper surface of the insulatingfilm GI except the narrow periphery thereof within the pixel area.

The side of the pixel electrode PX that is adjacent to the thin filmtransistor TFT is formed to avoid the region in which the thin filmtransistor TFT is formed and to be superposed on and connected to theextended portion of the source electrode SD2.

The sides of the pixel electrode PX that are adjacent to the respectivedrain signal lines DL are formed so that the outlines of the respectivesides are superposed on the central axes (or approximately central axes)of the respective light shield films SKD.

Each of the light shield films SKD mainly functions to block light leakdue to an electric field generated between the pixel electrode PX and anadjacent one of the drain signal lines DL as well as light due to thealignment disorder of the liquid crystal due to an electric fieldgenerated from the adjacent one of the drain signal lines DL in theperiphery of the pixel electrode PX.

Furthermore, the pixel electrode PX is formed to slightly overlap thegate signal line GL for driving the thin film transistor TFT and theother gate signal line GL (the gate signal line GL shown on the upperside of FIG. 1) which is disposed adjacent to the gate signal line GLwith the pixel electrode PX interposed therebetween.

The other gate signal line GL has an extended portion which projectsinto the pixel area as described previously, and the pixel electrode PXis superposed on the other gate signal line GL via the insulating filmGI in a comparatively large area.

The capacitance element Cadd which uses the insulating film GI as itsdielectric film is formed in the portion in which the pixel electrode PXand the other gate signal line GL are superposed on each other. Thecapacitance element Cadd serves, for example, the effect of enabling avideo signal supplied to the pixel electrode PX to be stored for acomparatively long time even when the thin film transistor TFT is turnedoff.

A protective film PSV which is made of, for example, SiN is formed onthe surface of the transparent substrate SUB1 formed in theabove-described manner.

This protective film PSV is provided for preventing the thin filmtransistor TFT from coming into direct contact with the liquid crystal,and apertures are formed in the areas of the protective film PSV inwhich the respective pixel electrodes PX are formed.

Specifically, the protective film PSV is formed so that the centralportion of the pixel electrode PX except the periphery thereof isexposed (refer to FIG. 2). In this construction, since the protectivefilm PSV is absent in the portion of the pixel area through which totransmit light, it is possible to prevent the absorption of light intothe protective film PSV.

An alignment layer ORI which covers the entire area of the display partAR is formed on the surface of the transparent substrate SUB1 formed inthe above-described manner (refer to FIG. 4), and serves to determinethe initial alignment direction of a liquid crystal LC which is indirect contact with the alignment layer ORI.

As shown in FIG. 4, the black matrix BM is formed on theliquid-crystal-LC-side surface of the transparent substrate SUB2 so asto partition each of the pixel areas from the adjacent one. Colorfilters FIL having colors corresponding to the respective pixel areasare individually formed in apertures formed in the portions of the blackmatrix BM that correspond to the respective pixel areas.

Another alignment layer ORI which covers the entire area of the displaypart AR is formed on the surface of the transparent substrate SUB2formed in the above-described manner, and serves to determine theinitial alignment direction of the liquid crystal LC which is in directcontact with the alignment layer ORI.

<<Gate Signal Line GL>>

Each of the gate signal lines GL is made of a two-layer structure, andits lower layer is made of an ITO (Indium-Tin-Oxide) film g1, while itsupper layer is made of a Mo layer g2, as shown in FIG. 2 or 3 by way ofexample.

In the recent trend toward increased sizes of liquid crystal displaypanels, it has been desired to reduce the resistivity of each of thegate signal lines GL, and the Mo layer g2 is selected as the mainmaterial of the gate signal lines GL. However, if the Mo layer g2 isused as a single layer, the adhesion of the Mo layer g2 to thetransparent substrate SUB1 which is a base substrate becomesinsufficient. For this reason, the ITO film g1 is used as anintermediate layer.

By subjecting each of the gate signal lines GL having the two-layerstructure to selective etching which allows for the etching rates of therespective layers (as will be described later), tapered surfaces whichbecome gradually more open toward the transparent substrate SUB1 can beformed on the side walls of each of the gate signal lines GL, whereby itis possible to prevent so-called climb-over disconnection of the drainsignal lines DL which climb over the gate signal lines GL and it is alsopossible to prevent so-called climb-over damage to the protective filmPSV.

During the formation of the gate terminal parts GTM of the respectivegate signal lines GL, there may be a case where selectivity to the Molayer g2 cannot be ensured when contact holes are to be formed bydry-etching the protective film PSV and the insulating film GI. However,since the ITO film g1 remains as a so-called stopper, the gate terminalparts GTM can be formed with good reliability.

FIGS. 7A to 7C are process diagrams showing one embodiment of a methodof forming one of the above-described gate signal lines GL.

First, as shown in FIG. 7A, the ITO film g1 is formed on the mainsurface of the transparent substrate SUB1, and the Mo layer g2 is formedon the upper surface of the ITO film g1. Then, a photoresist film PRESis formed on the surface of the Mo layer g2, and this photoresist filmPRES is selectively exposed by using a photomask (not shown). Afterthat, the photoresist film PRES is developed so that the photoresistfilm PRES is left in an area in which to form the gate signal line GL.

Then, the photoresist film PRES is used as a mask to selectively etchthe portion of the Mo layer g2 that is exposed from this mask. Theetching solution used in this step is selected from, for example, amixed acid containing phosphoric acid and nitric acid and a mixedsolution of cerium nitrate and nitric acid. In this case, taperedsurfaces which become gradually more open toward the transparentsubstrate SUB1 are respectively formed on the side walls of theremaining Mo layer g2.

Then, the photoresist film PRES is again used as a mask to selectivelyetch the portion of the ITO film g1 that is exposed from this mask. Theetching solution used in this step is, for example, aqua regia (a mixedsolution of hydrochloric acid and nitric acid). In this case, taperedsurfaces which become gradually more open toward the transparentsubstrate SUB1 are respectively formed on the side walls of theremaining Mo layer g1.

After that, the photoresist film PRES is removed, whereby the gatesignal line GL is formed on the transparent substrate SUB1. This gatesignal line GL has side walls which respectively have gently taperedsurfaces which become gradually more open toward the transparentsubstrate SUB1, whereby it is possible to fully prevent a layer to bestacked in a later step from undergoing damages such as cuttings due tosteps.

<<Drain Signal Line DL>>

A cross section of one of the drain signal lines DL is clearly shown inFIG. 4. The shown drain signal line DL is formed in parallel with thethin film transistors TFT formed by a resist reflow method which will bedescribed later, and is made of a stacked structure in which thesemiconductor layer AS made of a-Si, the high-concentration layer d₀formed on the surface of the semiconductor layer AS, and a Mo layer d₁are stacked in that order.

Accordingly, as shown in FIG. 4, tapered surfaces which become graduallymore open toward the transparent substrate SUB1 are respectively formedon the side walls of the drain signal line DL, and steps arerespectively formed midway on these tapered surfaces, specifically, inthe portion of the semiconductor layer AS that underlies thehigh-concentration layer do.

Therefore, it is possible to reliably prevent the drain signal line DLfrom causing so-called climb-over damage to the protective film PSV andthe alignment layer ORI.

The climb-over damage occurring in this case causes the problem that acrack or the like occurs in the protective film PSV in the vicinity ofeither of the side walls of the drain signal line DL and the material ofthe drain signal line DL is ionized through this crack and solved out inthe liquid crystal, thus changing the resistivity of the liquid crystal.

<<Thin Film Transistor TFT>>

FIG. 2 is a view showing a cross section of one of the thin filmtransistors TFT. The shown thin film transistor TFT is formed by using aso-called resist reflow method which will be described later.

The gate signal line GL which constitutes part of the gate electrode ofthe thin film transistor TFT has side walls formed in a tapered shapewhich becomes gradually more open toward the transparent substrate SUB1,whereby the insulating film G1, the drain electrode SD1 and the sourceelectrode SD2 are stacked in that portion can be prevented from beingdamaged by steps.

The pixel electrode PX electrically connected to the source electrodeSD2 of the thin film transistor TFT is formed to be directly stacked onthe source electrode SD2. Accordingly, the protective film PSV whichprevents the thin film transistor TFT from coming in direct contact withthe liquid crystal LC is formed to overlie the pixel electrode PX.

Specifically, the pixel electrode PX is positioned as a layer whichunderlies the protective film PSV, whereby the formation of a contacthole to the protective film PSV is avoided with respect to theelectrical connection between the source electrode SD2 of the thin filmtransistor TFT and the pixel electrode PX.

The thin film transistor TFT formed by a resist reflow method has sidewalls formed by a stacked structure in which the semiconductor layer AS,the high-concentration layer d₀ and the drain electrode SD1 or thesource electrode SD2 are stacked in that order. The respective sidewalls are formed to have tapered surfaces which become gradually moreopen toward the transparent substrate SUB1, and steps are respectivelyformed midway on these tapered surfaces, specifically, in the portion ofthe semiconductor layer AS that underlies the high-concentration layerdo.

Therefore, it is possible to reliably prevent the drain signal line DLfrom causing so-called climb-over damage to the protective film PSV andthe alignment layer ORI.

In particular, the pixel electrode PX is made of a material which iscomparatively easily damaged at a location where the pixel electrode PXclimbs over a step. In addition, the pixel electrode PX must be formedto climb over the stacked structure and overlap the source electrodeSD2. However, since the steps are respectively formed on the taperedsurfaces of the side walls of the stacked structure, the pixel electrodePX can be fully prevented from undergoing climb-over damage due to thestep.

FIGS. 8A to 8F are process diagrams showing one embodiment of a methodof forming the above-described thin film transistor TFT.

First, as shown in FIG. 8A, after a gate signal line GL and aninsulating film GI have been formed, a semiconductor layer AS is formedon the surface of this insulating film GI and a high-concentration layerd₀ is formed on the surface of the semiconductor layer AS, and a Molayer d₁ is formed on the high-concentration layer d₀. In this case, thesemiconductor layer AS, the high-concentration layer d₀ and the Mo layerd₁ are continuously deposited in the same chamber.

As shown in FIG. 8B, a photoresist film PRES is formed on the surface ofthe Mo layer d₁, and the portions of the photoresist film PRES thatcorrespond to portions in which to form a drain signal line DL, a drainelectrode SD1 and a source electrode SD2 are left through selectiveexposure using a photomask.

As shown in FIG. 8C, the photoresist film PRES is used a mask, and theportion of the Mo layer d₁ that is exposed from this mask is selectivelyetched (for example, a mixed acid containing phosphoric acid and nitricacid or a mixed solution of cerium nitrate and nitric acid is selectedfor this etching), and further, the high-concentration layer d₀ isdry-etched. In this case, the surface of the semiconductor layer AS iscut to a slight extent.

Incidentally, the etching of the high-concentration layer d₀ is notlimited to only the above-described method, and after the selectiveetching of the Mo layer d₁ this Mo layer d₁ may be used as a mask toetch the high-concentration layer do.

As shown in FIG. 8D, the photoresist film PRES is made to reflow. Bythis treatment, the peripheral portions of the photoresist film PRES aresagged, and the sagged portions also function as a mask. The method ofreflowing the photoresist film PRES is, for example, baking, dissolutionin an organic solvent atmosphere or immersion in water.

In this step, the sags of the photoresist film PRES need to completelycover the portion (channel region) between the drain electrode SD1 andthe source electrode SD2. To this end, in the step shown in FIG. 8B, thepattern width of the photoresist film PRES in this portion needs to bemade as narrow as possible.

Furthermore, this photoresist film PRES is used as a mask to selectivelyetch the portion of the semiconductor layer AS that is exposed from thismask.

Then, the photoresist film PRES is removed. It is preferable to subjectthe photoresist film PRES to so-called MEA stripping after ashing hasbeen performed, because if the baking of the photoresist film PRES isperformed during the reflow thereof, the photoresist film PRES becomesdifficult to strip.

As shown in FIG. 8E, an ITO film ITO1 is formed, and a photoresist filmPRES is formed in a portion which corresponds to an area in which toform a pixel electrode PX (as well as a drain terminal part DTM). Thisphotoresist film PRES is used as a mask to remove the portion of the ITOfilm ITO1 that is exposed from this mask. After that, the photoresistfilm PRES is removed.

As shown in FIG. 8F, a protective film PSV is formed, and a photoresistfilm PRES is formed so that an aperture is formed in the central portionof the pixel area (including the region in which the thin filmtransistor TFT is formed) except the periphery of the pixel area.

This photoresist film PRES is used as a mask to remove the portion ofthe protective film PSV that is exposed from this mask. Incidentally, atthe same time that holes are formed in this protective film PSV, holesat the gate terminal parts GTM and the drain terminal parts DTM are alsoformed. After that, the photoresist film PRES is removed.

<<Capacitance Element Cadd>>

FIG. 3 is a view showing a cross section of one of the capacitanceelements Cadd. The pixel electrode PX is formed in such a manner that apart of the pixel electrode PX is superposed on a part of the gatesignal line GL with the insulating film GI interposed therebetween, andthe insulating film GI serves as the dielectric film of the capacitanceelement Cadd.

As described above, since the pixel electrode PX is formed to bepositioned as a layer underlying the protective film PSV, the dielectricfilm of the capacitance element Cadd does not use a two-layer structuremade of the protective film PSV and the insulating film GI, and isformed of only the insulating film GI.

For this reason, the capacitance value of the capacitance element Caddcan be set by the film thickness of the insulating film GI and the areaof superposition between the gate signal line GL and the pixel electrodePX, whereby the capacitance value of the capacitance element Cadd canreadily be set.

Steps due to the gate signal line GL easily appear on the surface of thedielectric film of the capacitance element Cadd owing to the fact thatthe dielectric film is formed of only the insulating film GI. However,the gate signal line GL has a two-layer structure which includes the ITOfilm g1 as its lower layer and the Mo layer g2 as its upper layer, andthe side walls of the gate signal line GL is formed in the shape of agentle taper, whereby in the case where part of the pixel electrode PXis superposed on the gate signal line GL, it is possible to fullyprevent the pixel electrode PX from being damaged at a location wherethe pixel electrode PX climbs over a step.

<<Black Matrix BM>>

A cross section of the black matrix BM is shown in FIG. 4. FIG. 4 showsthat the black matrix BM is formed to cover only the drain signal lineDL, but the black matrix BM is also formed to cover the gate signallines GL and the thin film transistors TFT.

This construction is intended to improve contrast and to prevent avariation in the characteristics of the thin film transistor TFT due toirradiation with external light.

The light shield films SKD which are formed at the same time as the gatesignal lines GL are formed on the opposite sides of the drain signalline DL, and the black matrix BM which covers the drain signal line DLis formed so that the widthwise opposite ends of the black matrix BM arerespectively positioned over the light shield films SKD.

<<Gate Terminal Part GTM>>

FIG. 5 is a view showing a cross section of one of the gate terminalparts GTM. The shown gate terminal part GTM is formed by openingapertures, respectively, in the protective film PSV and the insulatingfilm GI in that order (by selective etching using dry etching) andexposing an extending end of the gate signal line GL. The opening of theapertures is performed at the same time as the opening of apertures inthe protective film PSV in the respective pixel areas.

As is apparent from FIG. 5, the gate signal line GL which has beenformed of the ITO film g1 as the lower layer and the Mo layer g2 as theupper layer is formed with the overlying Mo layer g2 being removed atthe gate terminal part GTM. This is because the Mo layer g2 for which noselection ratio can be ensured is etched while the apertures are beingopened in the protective film PSV and the insulating film GI by dryetching.

However, the underlying ITO film g1 having the function of a stopper ofthe etching is left, and the function of the gate terminal part GTM isfully realized by the ITO film g1. In addition, the ITO film g1 is madeof a material which cannot easily be oxidized, whereby it is possible toform, for example, the gate terminal part GTM having reliability inresistance to electrolytic corrosion.

<<Drain Terminal Part DTM>>

FIG. 6 is a view showing a cross section of one of the drain terminalparts DTM. The shown drain terminal part DTM is formed by opening anaperture in the protective film PSV (by selective etching) and exposingan extending end of the drain signal line DL. The opening of theaperture is performed at the same time as the opening of apertures inthe protective film PSV in the respective pixel areas.

In this construction, the ITO film ITO1 is formed to cover the drainsignal line DL at the drain terminal part DTM. This ITO film ITO1 isformed at the same time that the pixel electrode PX is formed. The ITOfilm ITO1 is formed so that the occurrence of electrolytic corrosion canbe prevented.

As described above, the drain signal line DL is formed in parallel withthe thin film transistors TFT formed by the resist reflow method, and ismade of a stacked structure in which the semiconductor layer AS, thehigh-concentration layer d₀ and the Mo layer d₁ are stacked in thatorder. The side walls of the drain signal line DL is formed in the shapeof a gentle taper.

Accordingly, in the case where the drain signal line DL is covered withthe ITO film ITO1 at the drain terminal part DTM, it is possible tosolve a problem such as the cuttings of the ITO film ITO1 due to steps.

<<Manufacturing Method>>

FIGS. 9A to 9H are process diagrams showing one embodiment of amanufacturing method for the above-described liquid crystal displaydevice.

FIGS. 9A to 9H are views showing the process diagrams for forming theportion of the gate terminal part GTM, together with the processdiagrams shown in FIGS. 8A to 8F.

FIG. 9A corresponds to FIG. 8A, FIG. 9B corresponds to FIG. 8B, FIG. 9Ccorresponds to FIG. 8C, FIG. 9D corresponds to FIG. 8D, FIG. 9Ecorresponds to FIG. 8E, and FIG. 9G corresponds to FIG. 8F.

A series of processes is as shown in the Table of FIG. 10. As isapparent from this table, the number of photo-processes can be reducedto four, specifically, the patterning of the gate signal lines GL, thepatterning of the drain signal lines DL (the drain electrodes and thesource electrodes), the patterning of the pixel electrodes PX and thepattering of the protective film PSV.

Embodiment 2.

In the above description of Embodiment 1, reference has been made to theliquid crystal display device in which the thin film transistors TFT areformed by using the resist reflow method. However, the invention canalso be applied to a liquid crystal display device in which its thinfilm transistors TFT are formed by using a so-called half-exposuremethod.

Embodiment 2 is the same as Embodiment 1 except the followingconstruction.

<<Thin Film Transistor TFT>>

FIG. 11 is a view showing a cross section of a thin film transistor TFTformed by using a so-called half-exposure method.

The thin film transistor TFT has side walls formed by a stackedstructure in which a semiconductor layer AS, a high-concentration layerd₀ and a Mo layer d₁ are stacked in that order, and the respective sidewalls are formed to have gently tapered surfaces which become graduallymore open toward the transparent substrate SUB1.

FIGS. 15A to 15E are process diagrams showing one embodiment of a methodof forming the thin film transistor TFT.

First, as shown in FIG. 15A, after a gate signal line GL and aninsulating film GI have been formed, the semiconductor layer AS isformed on the surface of this insulating film GI and thehigh-concentration layer d₀ is formed on the surface of thesemiconductor layer AS, and the Mo layer d₁ is formed on thehigh-concentration layer d₀. In this case, the semiconductor layer AS,the high-concentration layer d₀ and the Mo layer d₁ are continuouslydeposited in the same chamber.

As shown in FIG. 15B, a photoresist film PRES is formed on the surfaceof the Mo layer d₁, and selective exposure using a photomask isperformed. The photomask used in this case is either a mask having agrid structure or a mask fabricated by controlling the film thickness ofa semi-transparent film such as MoSi, and the photoresist file PRES leftby using the photomask in portions which correspond to portions in whichto form a drain signal line DL, a drain electrode SD1 and a sourceelectrode SD2 and in a portion which corresponds to the portion (channelportion) between the drain electrode SD1 and the source electrode SD2.In this case, the film thickness of the photoresist film PRES over thechannel portion is made smaller than the film thickness of thephotoresist film PRES over the other regions.

Specifically, in the channel portion, resist conditions are controlledso that the time of completion of etching of the photoresist, the Molayer d₁ and the high-concentration layer d₀ becomes (approximately) thesame as the time of completion of etching of the Mo layer d₁, thehigh-concentration layer d₀ and the semiconductor layer AS.

As shown in FIG. 15C, the photoresist film PRES is used a mask, and theportion of the Mo layer d₁ that is exposed from this mask is selectivelyetched (for example, a mixed acid containing phosphoric acid and nitricacid or a mixed solution of cerium nitrate and nitric acid is selectedfor this etching), and further, the high-concentration layer d₀ and thesemiconductor layer AS are dry-etched. In this case, in the channelportion, the Mo layer d₁ and the high-concentration layer d₀ as well asthe semiconductor layer AS are etched, but the surface of thesemiconductor layer AS is only cut to a slight extent.

Incidentally, the etching of the high-concentration layer d₀ is notlimited to only the above-described method, and after the selectiveetching of the Mo layer d₁, this Mo layer d₁ may be used as a mask toetch the high-concentration layer d₀.

After that, the photoresist film PRES is removed.

As shown in FIG. 15D, an ITO film ITO1 is formed, and a photoresist filmPRES is formed in a portion which corresponds to an area in which toform a pixel electrode PX (as well as a drain terminal part DTM). Thisphotoresist film PRES is used as a mask to remove the portion of the ITOfilm ITO1 that is exposed from this mask. After that, the photoresistfilm PRES is removed.

As shown in FIG. 15E, a protective film PSV is formed, and a photoresistfilm PRES is formed so that an aperture is formed in the central portionof the pixel area (including the region in which the thin filmtransistor TFT is formed) except the periphery of the pixel area.

This photoresist film PRES is used as a mask to remove the portion ofthe protective film PSV that is exposed from this mask. Incidentally, atthe same time that holes are formed in this protective film PSV, holesat the gate terminal parts GTM and the drain terminal parts DTM are alsoformed. After that, the photoresist film PRES is removed.

<<Drain Signal Line DL>>

A cross section of one of drain signal lines DL is shown in FIG. 12. Theshown drain signal line DL is made of a stacked structure in which thesemiconductor layer AS, the high-concentration layer d₀ and the Mo layerd₁ are stacked in that order, and is formed in parallel with the thinfilm transistors TFT formed by the half-exposure method. Accordingly, asshown in FIG. 12, tapered surfaces which become gradually more opentoward the transparent substrate SUB1 are respectively formed on theside walls of the drain signal line DL.

<<Gate Terminal Part GTM>>

A cross section of one of the gate terminal parts GTM is shown in FIG.13. The shown gate signal line GTM is formed similarly to the case ofEmbodiment 1.

<<Drain Terminal Part DTM>>

A cross section of one of the drain terminal parts DTM is shown in FIG.14. The shown drain terminal part DTM is made of a stacked structure inwhich the semiconductor layer AS, the high-concentration layer d₀ andthe Mo layer d₁ are stacked in that order, and tapered surfaces whichbecome gradually more open toward the transparent substrate SUB1 arerespectively formed on the side walls of the drain signal line DL.

At the drain terminal part DTM, the ITO film ITO1 is formed to extend tothe side walls of the drain signal line DL. The ITO film ITO1 is formedat the same time that the pixel electrode PX is formed for preventingthe electric corrosion.

The drain terminal part DTM is formed by opening an aperture in theprotective film PSV and exposing the ITO film ITO1.

<<Manufacturing Method>>

FIGS. 16A to 16G are process diagrams showing one embodiment of amanufacturing method for the above-described liquid crystal displaydevice.

FIGS. 16A to 16G are views showing the process diagrams for forming theportion of the gate terminal part GTM, together with the processdiagrams shown in FIGS. 15A to 15E.

FIG. 16A corresponds to FIG. 15B, FIG. 16C corresponds to FIG. 15C, FIG.16D corresponds to FIG. 15D, and FIG. 9F corresponds to FIG. 15E.

A series of processes is as shown in the Table of FIG. 17. As isapparent from this table, the number of photo-processes can be reducedto four, specifically, the patterning of the gate signal lines GL, thepatterning of the drain signal lines DL (the drain electrodes and thesource electrodes), the patterning of the pixel electrodes PX and thepattering of the protective film PSV.

Embodiment 3.

The invention can also be applied to a liquid crystal display device inwhich its thin film transistors TFT are formed by using a so-called ITOmask method.

Embodiment 3 is the same as Embodiment 1 except the followingconstruction.

<<Thin Film Transistor TFT>>

FIG. 18 is a view showing a cross section of a thin film transistor TFTformed by using an ITO mask method.

In the thin film transistor TFT, the ITO film ITO1 of the pixelelectrode PX is directly superposed on not only the entire surface ofthe source electrode SD2, but also the surface of the drain signal lineDL (and the drain electrode SD1).

Similarly to the case of the other embodiments, the side walls of astacked structure in which the semiconductor layer AS, thehigh-concentration layer d₀ and the Mo layer d₁ are stacked in thatorder are respectively formed to have gently tapered surfaces whichbecome gradually more open toward the transparent substrate SUB1.

FIGS. 22A to 22G are process diagrams showing one embodiment of a methodof forming the thin film transistor TFT.

First, as shown in FIG. 22A, after a gate signal line GL and aninsulating film GI have been formed, the semiconductor layer AS isformed on the surface of this insulating film GI and thehigh-concentration layer d₀ is formed on the surface of thesemiconductor layer AS, and the Mo layer d₁ is formed on thehigh-concentration layer d₀. In this case, the semiconductor layer AS,the high-concentration layer d₀ and the Mo layer d₁ are continuouslydeposited in the same chamber.

As shown in FIG. 22B, a photoresist film PRES is formed on the surfaceof the Mo layer d₁ and through selective exposure using a photomask, thephotoresist film PRES is left in portions which correspond to portionsin which to form a drain signal line DL, a drain electrode SD1 and asource electrode SD2 and in a portion which corresponds to the region(channel portion) between the drain electrode SD1 and the sourceelectrode SD2.

Then, the photoresist film PRES is used a mask, and the portion of theMo layer d₁ that is exposed from this mask, the high-concentration layerd₀ and the semiconductor layer AS are selectively etched in that order.After that, the photoresist film PRES is removed.

As shown in FIG. 22C, an ITO film ITO is formed, and a photoresist filmPRES is formed on the upper surface of the ITO film ITO. The photoresistfilm PRES is left in a portion which corresponds to an area in which toform the drain signal line DL, the drain electrode SD1 and the pixelelectrode PX.

As shown in FIG. 22D, the photoresist film PRES is used as a mask toselectively etch the portion of the ITO film ITO that is exposed fromthis mask. After that, the photoresist film PRES is removed.

As shown in FIG. 22E, the ITO film ITO is used as a mask to selectivelyetch the portion of the Mo layer d₁ that is exposed from this mask, andfurthermore, the high-concentration layer d₀ is selectively etched.Thus, the surface of the semiconductor layer AS is exposed.

As shown in FIG. 22F, a protective film PSV is formed, and a photoresistfilm PRES is formed so that an aperture is formed in the central portionof the pixel area (including the region in which the thin filmtransistor TFT is formed) except the periphery of the pixel area.

This photoresist film PRES is used as a mask to remove the portion ofthe protective film PSV that is exposed from this mask. Incidentally, atthe same time that holes are formed in this protective film PSV, holesat the gate terminal parts GTM and the drain terminal parts DTM are alsoformed. After that, the photoresist film PRES is removed, as shown inFIG. 22G.

<<Drain Signal Line DL>>

A cross section of one of drain signal lines DL is shown in FIG. 19. Theshown drain signal line DL is made of a stacked structure in which thesemiconductor layer AS, the high-concentration layer d₀, the Mo layer d₁and the ITO film ITO1 are stacked in that order.

<<Gate Terminal Part GTM>>

A cross section of one of the gate terminal parts GTM is shown in FIG.20. The shown gate terminal part GTM has a construction similar to thatof the gate terminal part GTM of each of Embodiments 1 and 2.

<<Drain Terminal Part DTM>>

A cross section of one of the drain terminal parts DTM is shown in FIG.21. At the drain terminal part DTM, since the drain signal line DL ismade of the above-described stacked structure, the ITO film ITO1 isexposed at the bottom of the aperture of the protective film PSV so thatit is possible to obtain a construction having reliability in resistanceto electrolytic corrosion.

<<Manufacturing Method>>

FIGS. 23A to 23G are process diagrams showing one embodiment of amanufacturing method for the above-described liquid crystal displaydevice.

FIGS. 23A to 23G are views showing the process diagrams for forming theportion of the gate terminal part GTM, together with the processdiagrams shown in FIGS. 22A to 22G.

FIG. 23A corresponds to FIG. 22B, FIG. 23C corresponds to FIG. 22C, FIG.23D corresponds to FIG. 22D, FIG. 23E corresponds to FIG. 22E, FIG. 23Fcorresponds to FIG. 22F, and FIG. 23G corresponds to FIG. 22G.

A series of processes is as shown in the Table of FIG. 24. As isapparent from this table, the number of photo-processes can be reducedto four, specifically, the patterning of the gate signal lines GL, thepatterning of the pixel electrodes PX (the drain signal lines DL), thepatterning of the drain electrodes and the source electrodes, and thepattering of the protective film PSV.

Embodiment 4.

In the above description of each of the embodiments, reference has beenmade to a so-called vertical electric field type of liquid crystaldisplay device in which a pixel electrode is formed in each pixel areaon its transparent substrate SUB1 and a counter electrode common to eachpixel area is formed on its glass substrate SUB2 so that the opticaltransmissivity of its liquid crystal is controlled by an electric fieldgenerated between each of these electrodes approximately perpendicularlyto the substrates.

However, the invention is not limited to such a vertical electric fieldtype of liquid crystal display device, and can also be applied to aso-called in-plane-switching type of liquid crystal display device inwhich a pixel electrode and a counter electrode are formed in each pixelarea on its transparent substrate SUB1 so that the opticaltransmissivity of its liquid crystal is controlled by an electric fieldcomponent generated between the pixel electrode and the counterelectrode approximately in parallel with the transparent substrate SUB1.

FIG. 25 is a plan view showing the construction of a pixel on thein-plane-switching type of liquid crystal display device, andcorresponds to FIG. 1. Incidentally, FIG. 26 shows a cross-sectionalview taken along line 26-26 of FIG. 25 and FIG. 27 shows across-sectional view taken along line 27-27 of FIG. 25.

The construction shown in FIG. 25 differs from that shown in FIG. 1 aswill be described below.

Referring to FIG. 25, first of all, a plurality of pixel electrodes PXand a plurality of counter electrodes CT are disposed in the form ofstripes which are extended in parallel with drain signal lines DL, andthe pixel electrodes PX and the counter electrodes CT are formed to bealternately arranged.

In Embodiment 4, two pixel electrodes PX and three counter electrodes CTare disposed, and these two pixel electrodes PX and three counterelectrodes CT are alternately arranged so that two of the counterelectrodes CT are respectively disposed on the opposite extreme sides,that is to say, adjacently to the drain signal lines DL.

The respective counter electrodes CT disposed on the opposite extremesides have shielding functions for preventing the pixel electrodes PXfrom being influenced by electric fields generated from the adjacentdrain signal lines DL, and are formed to be wider than the otherelectrodes.

The counter electrodes CT are formed at the same time as gate signallines GL, and are formed of the same material as the gate signal linesGL. Accordingly, the counter electrodes CT are each made of a two-layerstructure having a lower layer formed of an ITO film and an upper layerformed of a Mo layer.

The three counter electrodes CT are connected in common by being formedintegrally with a counter voltage signal line CL which is formed inparallel with the gate signal lines GL in the central portion of thepixel area. A counter voltage signal is supplied to the counterelectrodes CT via the counter voltage signal line CL. Accordingly, thiscounter voltage signal line CL is also made of a two-layer structurehaving a lower layer formed of an ITO film and an upper layer formed ofa Mo layer.

This counter voltage signal line CL is formed in common with pixelsjuxtaposed in the x direction as viewed in FIG. 25, and is connected incommon to the other counter voltage signal lines CL of pixels arrangedin the y direction as viewed in FIG. 25. The counter voltage signal lineCL is supplied with the counter voltage signal from a common terminalpart CTM which is formed at a common connection line extended to theoutside of a display part.

The pixel electrodes PX are formed as a layer different from the counterelectrodes CT (the counter voltage signal line CL) with an insulatingfilm GI interposed between the pixel electrodes PX and the counterelectrodes CT.

The two pixel electrodes PX are formed in a pattern in which they areconnected to each other over the counter voltage signal line CL. In thisconnection portion, a storage capacitance Cstg which uses the insulatingfilm GI as its dielectric is formed between the pixel electrodes PX andthe counter voltage signal line CL.

The one of the two pixel electrodes PX that is closer to the thin filmtransistor TFT is extended at one end to the upper surface of asemiconductor layer AS of the thin film transistor TFT, and constitutesthe source electrode of the thin film transistor TFT.

This thin film transistor TFT is formed by the resist reflow methoddescribed previously in, for example, Embodiment 1.

<<Capacitance Element Cstg>>

FIG. 26 is a view showing a cross section of the capacitance elementCstg. A part of the pixel electrode PX is superposed on a part of thecounter voltage signal line CL with the insulating film GI interposedtherebetween, and the insulating film GI serves as the dielectric filmof the capacitance element Cstg.

Since the pixel electrode PX is formed to be positioned as a layerunderlying the protective film PSV, the dielectric film of thecapacitance element Cstg does not use a two-layer structure made of theprotective film PSV and the insulating film GI, and is formed of onlythe insulating film GI.

For this reason, the capacitance value of the capacitance element Cstgcan be set by the film thickness of the insulating film GI and the areaof superposition between the counter voltage signal line CL and thepixel electrode PX, whereby the capacitance value of the capacitanceelement Cstg can readily be set.

<<Counter Electrode CT>>

Cross sections of the counter electrodes CT are shown in FIG. 27.

By subjecting each of the counter electrodes CT having the two-layerstructure to selective etching which allows for the etching rates of therespective layers, tapered surfaces which become gradually more opentoward the transparent substrate SUB1 can be formed on the side walls ofeach of the counter electrodes CT, whereby it is possible to preventdamages such as cracks from occurring in the insulating film GI at alocation where the insulating film GI climbs over any of the counterelectrodes CT, and it is also possible to prevent the distribution ofelectric fields between the counter electrodes CT and the pixelelectrodes PX.

<<Drain Signal Line DL>>

Cross sections of the drain signal lines DL are shown in FIG. 27. Eachof the drain signal lines DL is formed in parallel with the thin filmtransistors TFT formed by a resist reflow method, and is made of astacked structure in which a semiconductor layer AS, ahigh-concentration layer d₀ and a Mo layer d₁ are stacked in that order.Tapered surfaces which become gradually more open toward the transparentsubstrate SUB1 are respectively formed on the side walls of each of thedrain signal lines DL, and steps are respectively formed midway on thesetapered surfaces, specifically, in the portion of the semiconductorlayer AS that underlies the high-concentration layer d₀. Incidentally,the thin film transistors TFT may be formed by the above-describedhalf-exposure method instead of the resist reflow method.

Therefore, it is possible to fully prevent climb-over damages fromoccurring in the protective film PSV and the alignment layer ORI whichare formed to overlie the drain signal lines DL.

<<Gate Terminal Part GTM, Common Terminal Part CTM>>

Each gate terminal part GTM has a construction similar to that of thegate terminal part GTM described previously in Embodiment 1 (FIG. 5).The common terminal part CTM has a two-layer structure formed of thesame material as and in the same layer as the gate signal lines GL, andhas a construction similar to that of the gate terminal part GTM.

<<Drain Terminal Part DTM>>

Each of drain terminal parts DTM has a construction similar to that ofthe drain terminal part DTM described previously in Embodiment 1 (FIG.6).

<<Thin Film Transistor TFT>>

Each of the thin film transistors TFT has a construction similar to thatof the thin film transistor TFT described previously in Embodiment 1(FIG. 2).

Embodiment 5.

FIG. 28 is a plan view showing another embodiment of a pixel on thein-plane-switching type of liquid crystal display device.

Referring to FIG. 28, first of all, pixel electrodes PX and a counterelectrode CT are respectively formed of transparent conductive layerssuch as ITO films, and the counter electrode CT is superposed on thepixel electrodes PX and is formed in nearly the entire area of the pixelarea.

Specifically, the counter electrode CT is formed on the entire centralportion of the pixel area except the narrow periphery thereof on thesurface of the transparent substrate SUB1.

A counter voltage signal is supplied to this counter electrode CT via acounter voltage signal line CL which is formed to run through thecentral portion of the pixel area in the x direction as viewed in FIG.28.

The counter voltage signal line CL is formed directly on the counterelectrode CT, and is formed at the same time that the gate signal linesGL are formed. Accordingly, the counter voltage signal line CL is madeof a two-layer structure having a lower layer formed of an ITO film andan upper layer formed of a Mo layer.

The pixel electrodes PX are formed on an insulating film GI which coversthe counter electrode CT (the counter voltage signal line CL). The pixelelectrodes PX are disposed in, for example, a stripe-shaped patternwhich is extended in parallel with drain signal lines DL, and arejuxtaposed in a direction perpendicular to the drain signal lines DL.

The pixel electrodes PX are connected in common at an end connected tothe thin film transistor TFT, and is extended to the surface of asemiconductor layer AS of the thin film transistor TFT and constitutes asource electrode SD2 of the thin film transistor TFT.

This thin film transistor TFT is formed by a resist reflow methodsimilarly to the case of Embodiment 1.

<<Counter Electrode CT>>

A cross section of the counter voltage signal line CL is shown in FIG.30.

By subjecting the counter voltage signal line CL having the two-layerstructure to selective etching which allows for the etching rates of therespective layers, tapered surfaces which become gradually more opentoward the transparent substrate SUB1 can be formed on the side walls ofthe counter voltage signal line CL, whereby it is possible to preventclimb-over damages occurring in the insulating film GI.

In addition, since the counter voltage signal line CL is formed on theupper surface of the counter electrode CT made of an ITO film and thelower layer of the counter voltage signal line CL is made of an ITOfilm, the adhesion of the counter voltage signal line CL to the counterelectrode CT can be ensured.

<<Drain Signal Line DL>>

Cross sections of the drain signal lines DL are shown in FIG. 29. Eachof the drain signal lines DL is formed in parallel with the thin filmtransistors TFT formed by a resist reflow method, and is made of astacked structure in which the semiconductor layer AS, thehigh-concentration layer d₀ and the Mo layer d₁ are stacked in thatorder. Tapered surfaces which become gradually more open toward thetransparent substrate SUB1 are respectively formed on the side walls ofeach of the drain signal lines DL, and steps are respectively formedmidway on these tapered surfaces, specifically, in the portion of thesemiconductor layer AS that underlies the high-concentration layer d₀.Incidentally, the thin film transistors TFT may be formed by theabove-described half-exposure method instead of the resist reflowmethod.

Therefore, it is possible to fully prevent climb-over damages fromoccurring in the protective film PSV and the alignment layer ORI whichare formed to overlie the drain signal lines DL.

<<Gate Terminal Part GTM, Common Terminal Part CTM>>

Each of gate terminal parts GTM and a common terminal part CTM has aconstruction similar to that of the gate terminal part GTM describedpreviously in Embodiment 1 (FIG. 5).

<<Drain Terminal Part DTM>>

Each of drain terminal parts DTM has a construction similar to that ofthe drain terminal part DTM described previously in Embodiment 1 (FIG.6).

<<Thin Film Transistor TFT>>

Each of the thin film transistors TFT has a construction similar to thatof the thin film transistor TFT described previously in Embodiment 1(FIG. 2).

Embodiment 6.

FIG. 31 is a plan view showing another embodiment of a pixel on thein-plane-switching type of liquid crystal display device, andcorresponds to FIG. 25.

FIG. 31 shows the construction of a pixel formed through a half-exposuremethod. The shown gate signal lines GL has a two-layer structure made ofan ITO film and a Mo layer similarly to the case of each of theabove-described embodiments, but this construction is not shown in FIG.31.

<<Storage Capacitance Cstg>>

A cross section of one of storage capacitances Cstg is shown in FIG. 32.

The storage capacitance Cstg is formed in such a manner that the pixelelectrode PX is superposed on the upper surface of the counter voltagesignal line CL with the insulating film GI interposed therebetween.

The counter voltage signal line CL has a two-layer structure whichincludes the ITO film g1 as its lower layer and the Mo layer g2 as itsupper layer. The pixel electrode PX is made of a stacked structure inwhich the semiconductor layer AS, the high-concentration layer d₀, theMo layer d₁ and the ITO film ITO1 are stacked in that order.

<<Pixel Electrode PX>>

Cross sections of the pixel electrodes PX are clearly shown in FIG. 33.

As described above, each of the pixel electrodes PX is made of a stackedstructure in which the semiconductor layer AS, the high-concentrationlayer d₀, the Mo layer d₁ and the ITO film ITO1 are stacked in thatorder.

In this case, although damages may be caused by steps due to the pixelelectrodes PX, the respective side walls of each of the pixel electrodesPX are formed to have gently tapered surfaces which become graduallymore open toward the transparent substrate SUB1. Accordingly, it ispossible to fully mitigate climb-over damages from occurring in theprotective film PSV and the alignment layer ORI which are formed tooverlie the drain signal lines DL.

<<Drain Signal Line DL>>

Cross sections of the drain signal lines DL are shown in FIG. 33.

Each of the drain signal lines DL is also made of a stacked structure inwhich the semiconductor layer AS, the high-concentration layer d₀, theMo layer d₁ and the ITO film are stacked in that order, similarly to thecase of the pixel electrodes PX.

For this reason, damages may be caused by steps due to the pixelelectrodes PX, but the respective side walls of each of the drain signallines DL are formed to have gently tapered surfaces which becomegradually more open toward the transparent substrate SUB1, whereby it ispossible to fully mitigate climb-over damages from occurring in theprotective film PSV and the alignment layer ORI which are formed tooverlie the drain signal lines DL.

<<Gate Terminal Part GTM>>

FIG. 35 is a view showing a cross section of one of gate terminal partsGTM. The shown gate terminal part GTM is formed by opening apertures,respectively, in the protective film PSV and the insulating film GI inthat order (by selective etching using dry etching) and exposing anextending end of the gate signal line GL.

As is apparent from FIG. 35, the gate signal line GL which has beenformed of the ITO film g1 as the lower layer and the Mo layer g2 as theupper layer is formed with the overlying Mo layer g2 being removed atthe gate terminal part GTM. This is because the Mo layer g2 for which noselection ratio can be ensured is etched while the apertures are beingopened in the protective film PSV and the insulating film GI by dryetching.

However, the underlying ITO film g1 having the function of a stopper ofthe etching is left, and the function of the gate terminal part GTM isfully realized by the ITO film g1. In addition, the ITO film g1 is madeof a material which cannot easily be oxidized, whereby it is possible toform, for example, the gate terminal part GTM having reliability inresistance to electrolytic corrosion.

<<Drain Terminal Part DTM>>

FIG. 36 is a view showing a cross section of one of drain terminal partsDTM. The shown drain signal line DL is made of a stacked structure inwhich the semiconductor layer AS, the high-concentration layer d₀, theMo layer d₁ and the ITO film ITO1 are stacked in that order.Accordingly, the drain terminal part DTM is formed by opening anaperture in the protective film PSV.

Since the ITO film ITO formed on the surface of the drain signal linesDL is exposed by opening the aperture in the protective film PSV, it isnot particularly necessary to form an ITO film for preventingelectrolytic corrosion.

<<Thin Film Transistor TFT>>

In Embodiment 6, the thin film transistors TFT are formed by ahalf-exposure method. FIGS. 37A to 37F are process diagrams showing oneembodiment of a method of forming the above-described thin filmtransistor TFT.

First, as shown in FIG. 37A, after a gate signal line GL and aninsulating film GI have been formed, a semiconductor layer AS is formedon the surface of this insulating film GI and a high-concentration layerd₀ is formed on the surface of the semiconductor layer AS, and a Molayer d₁ is formed on the high-concentration layer do, and furthermore,a ITO film ITO1 is formed on the Mo layer d₁. In this case, thesemiconductor layer AS, the high-concentration layer d₀, the Mo layerd₁and the ITO film ITO1 are continuously deposited in the same chamber.

As shown in FIG. 37B, a photoresist film PRES is formed on the surfaceof the ITO film ITO1, and selective exposure using a photomask isperformed. The photomask used in this case is either a mask having agrid structure or a mask fabricated by controlling the film thickness ofa semi-transparent film such as MoSi, and the photoresist film PRES isleft by using the photomask in portions which correspond to portions inwhich to form a drain signal line DL, a drain electrode SD1 and a sourceelectrode SD2 and in a portion which corresponds to the portion (channelportion) between the drain electrode SD1 and the source electrode SD2.In this case, the film thickness of the photoresist film PRES over thechannel portion is made smaller than the film thickness of thephotoresist film PRES over the other regions.

Specifically, in the channel portion, resist conditions are controlledso that the time of completion of etching of the photoresist, the Molayer d₁ and the high-concentration layer d₀ becomes (approximately) thesame as the time of completion of etching of the Mo layer d₁, thehigh-concentration layer d₀ and the semiconductor layer AS.

As shown in FIG. 37C, the photoresist film PRES is used a mask, and theportions of the ITO film ITO1, the Mo layer d₁, the high-concentrationlayer do and the semiconductor layer AS that are exposed from this maskare selectively etched (for example, a fluorine-containing gas such asSF₆ or CF₄ is selected as a gas for selective etching of thesemiconductor layer AS and the high-concentration layer d₀).

In this manner, the semiconductor layer AS which constitutes the thinfilm transistor TFT is etched in an island-like shape, but the channelportion is etched to at least the high-concentration layer d₀. Afterthat, the photoresist film PRES is removed.

As shown in FIG. 37D, a protective film PSV is formed. As shown in FIG.37E, a photoresist film PRES is formed on the surface of the protectivefilm PSV so that an aperture is formed in the central portion of thepixel area (including the region in which the thin film transistor TFTis formed) except the periphery of the pixel area.

As shown in FIG. 37F, this photoresist film PRES is used as a mask toremove the portion of the protective film PSV that is exposed from thismask. Incidentally, at the same time that holes are formed in thisprotective film PSV, holes at the gate terminal parts GTM and the drainterminal parts DTM are also formed. After that, the photoresist filmPRES is removed.

<<Manufacturing Method>>

FIGS. 38A to 38E are process diagrams showing one embodiment of amanufacturing method for the above-described liquid crystal displaydevice. FIGS. 38A to 38E are views showing the process diagrams forforming the portion of the gate terminal part GTM, together with theprocess diagrams shown in FIGS. 37A to 37F. FIG. 38A corresponds to FIG.37B, FIG. 38C corresponds to FIG. 37C, FIG. 38D corresponds to FIG. 37E,and FIG. 38E corresponds to FIG. 37F.

A series of processes is as shown in the Table of FIG. 39. As isapparent from this table, the number of photo-processes can be reducedto three, specifically, the patterning of the gate signal lines GL, thepatterning of the drain signal lines DL (the drain electrodes and thesource electrodes), and the pattering of the protective film PSV.

Embodiment 7.

FIG. 40 is a plan view showing another embodiment of a pixel on theliquid crystal display device according to the invention, andcorresponds to FIG. 28. Specifically, a counter electrode CT made of,for example, an ITO film is formed in the greater part of the pixelarea, and a plurality of pixel electrodes PX made of, for example, anITO film are formed to be juxtaposed in the form of stripes. InEmbodiment 7, the insulating film interposed between the counterelectrode CT and the pixel electrodes PX has a two-layer structure madeof an insulating film GI and a protective film PSV. A film transistorTFT is formed by a resist reflow method.

Incidentally, FIG. 41 shows a cross-sectional view taken along line41-41 of FIG. 40, FIG. 42 shows a cross-sectional view taken along line44-44 of FIG. 40, and FIG. 45 shows a cross-sectional view taken alongline 45-45 of FIG. 40.

<<Drain Signal Line DL>>

Cross sections of drain signal lines DL are shown in FIG. 41. Each ofthe drain signal lines DL is formed on the insulating film GI, and ismade of a stacked structure in which the semiconductor layer AS, thehigh-concentration layer d₀ and the Mo layer d₁ are stacked in thatorder.

Since the drain signal lines DL are formed in parallel with the thinfilm transistors TFT formed by the resist reflow method, the side wallsof each of the drain signal lines DL are respectively formed to havegently tapered surfaces (which become gradually more open toward thetransparent substrate SUB1) with steps being respectively formed on theopposite side surfaces of the semiconductor layer AS of each of thedrain signal lines DL.

<<Gate Signal Line GL>>

Cross sections of gate signal lines GL are shown in FIG. 42. Each of thegate signal lines GL is formed on the transparent substrate SUB1, and ismade of a stacked structure in which the ITO film g1 and the Mo layer g2are stacked in that order. Tapered surfaces which become gradually moreopen toward the transparent substrate SUB1 are respectively formed onthe side walls of each of the gate signal lines GL.

<<Counter Voltage Signal Line CL>>

A cross section of a counter voltage signal line CL is shown in FIG. 42.The counter voltage signal line CL is made of a stacked structure inwhich the ITO film g1 and the Mo layer g2 are stacked in that order,similarly to the gate signal lines GL. The ITO film g1 is formed to bewider than the Mo layer g2 so that the ITO film g1 can serve thefunction of the counter electrode CT.

As described above, although the counter electrode CT has a one-layerstructure, the counter voltage signal line CL has a two-layer structurein which a layer made of a material different from that of the counterelectrode CT is formed on a layer which constitutes the counterelectrode CT. The counter voltage signal line CL and the counterelectrode CT can be formed by one photo-process adopting a half-exposuremethod.

FIGS. 43A to 43D are views showing a process to be used in this case.First of all, as shown in FIG. 43A, a stacked structure in which, forexample, the ITO film g1 and the Mo layer g2 are stacked in that orderis formed on the surface of the transparent substrate SUB1.

Then, a photoresist film PRES is selectively formed on the surface ofthe stacked structure, but as shown in FIG. 43B, the photoresist filmPRES has different thicknesses in different areas. The formation of thephotoresist film having different thicknesses is achieved by effectingso-called half-exposure using either a photomask having a grid structureor a photomask fabricated by controlling the film thickness of asemi-transparent film such as MoSi.

After that, as shown in FIG. 43C, the overlying Mo layer g2 isselectively etched by using the photoresist film PRES as a mask. In thiscase, the photoresist film of larger film thickness is left in the formof a photoresist film of reduced film thickness, but the photoresistfilm of smaller film thickness vanishes.

Then, as shown in FIG. 43D, the underlying ITO film g1 is selectivelyetched by using the remaining photoresist film PRES as a mask. In thiscase, the Mo layer g2 lying on the side where the photoresist film isvanished is etched at this time.

<<Gate Terminal Part GTM>>

A cross section of one of gate terminal parts GTM is shown in FIG. 44.The shown gate terminal part GTM is formed by opening an aperture in theinsulating film GI and the protective film PSV which covers the gatesignal lines GL.

In this case, the Mo layer g2 which is the upper layer of the gatesignal lines GL is removed by opening the aperture, and the electrolyticcorrosion of the gate terminal part GTM can be far more effectivelyprevented by an ITO film which is formed in the aperture as well as in aportion around the aperture. Incidentally, the ITO film is formed at thesame time as the pixel electrodes PX.

<<Drain Terminal Part DTM>>

A cross section of one of drain terminal parts DTM is shown in FIG. 45.The drain terminal part DTM is formed by opening an aperture in theprotective film PSV which covers the drain signal lines DL.

In this case, the electrolytic corrosion of the gate terminal part GTMcan be far more effectively prevented by an ITO film which is formed inthe aperture as well as in a portion around the aperture. This ITO filmis also formed at the same time as the pixel electrodes PX.

<<Manufacturing Method>>

FIGS. 46A to 46D, FIGS. 47E to 47H and FIG. 481 are process diagramsshowing one embodiment of a manufacturing method for the above-describedliquid crystal display device, and show a portion which includes a thinfilm transistor TFT and a counter electrode CT.

First of all, as shown in FIG. 46A, after a gate signal line GL and aninsulating film GI have been formed, a semiconductor layer AS, ahigh-concentration layer d₀ and a Mo layer d₁ are formed on the surfaceof the insulating film GI in a stacked manner. In this case, thesemiconductor layer AS, the high-concentration layer d₀ and the Mo layerd₁ are continuously deposited in the same chamber.

As shown in FIG. 46B, a photoresist film PRES is formed on the surfaceof an ITO film, and selective exposure using a photomask is performed.The photomask used in this case is either a mask having a grid structureor a mask fabricated by controlling the film thickness of asemi-transparent film such as MoSi, and the photoresist film PRES isleft by using the photomask in portions which correspond to portions inwhich to form a drain signal line DL, a drain electrode SD1 and a sourceelectrode SD2 and in a portion which corresponds to the portion (channelportion) between the drain electrode SD1 and the source electrode SD2.In this case, the film thickness of the photoresist film PRES over thechannel portion is made smaller than the film thickness of thephotoresist film PRES over the other regions.

Specifically, in the channel portion, resist conditions are controlledso that the time of completion of etching of the photoresist, the Molayer d₁ and the high-concentration layer d₀ becomes (approximately) thesame as the time of completion of etching of the Mo layer d₁, thehigh-concentration layer d₀ and the semiconductor layer AS.

As shown in FIG. 46C, the photoresist film PRES is used a mask, and theportions of the Mo layer d₁, the high-concentration layer d₀ and thesemiconductor layer AS that are exposed from this mask are selectivelyetched (for example, a fluorine-containing gas such as SF₆ or CF₄ isselected as a gas for selective etching of the semiconductor layer ASand the high-concentration layer d₀).

In this manner, the semiconductor layer AS is exposed in the area otherthan the area in which to form the thin film transistor TFT, and thephotoresist film PRES is reduced in film thickness in its entire area.In the channel portion, the Mo layer d₁ is exposed from the photoresistfilm PRES.

As shown in FIG. 46D, etching is performed by using the remainingphotoresist film PRES as a mask. In this manner, the semiconductor layerAS is exposed in the area other than the area in which to form the thinfilm transistor TFT, and the insulating film GI is exposed.

In addition, in the channel portion, the Mo layer d₁ and thehigh-concentration layer d₀ are etched, and the semiconductor layer ASis exposed. As shown in FIG. 47E, a protective film PSV is formed.

As shown in FIG. 47F, a photoresist film PRES is formed on the uppersurface of the protective film PSV so that an aperture is formed in aportion in which to form a contact hole for the source electrode SD2 ofthe thin film transistor TFT. Then, the protective film PSV is etched byusing the photoresist film PRES as a mask. Incidentally, at the sametime that holes are formed in this protective film PSV, holes at thegate terminal parts GTM and the drain terminal parts DTM are alsoformed. After that, the photoresist film PRES is removed.

As shown in FIG. 47G, the contact hole is formed in the protective filmPSV, and a part of the source electrode SD2 of the thin film transistorTFT is exposed from this contact hole. As shown in FIG. 47H, an ITO filmis formed on the protective film PSV, and a photoresist film PRES isformed on the ITO film over an area in which to form a pixel electrodesPX and an extending portion of the pixel electrodes PX to be connectedto the source electrode SD2 of the thin film transistor TFT. As shown inFIG. 481, the ITO film is etched by using the photoresist film PRES as amask, and then the photoresist film PRES is removed.

Embodiment 8.

FIG. 49 is a plan view showing another embodiment of a pixel on theliquid crystal display device according to the invention, andcorresponds to FIG. 40. The construction shown in FIG. 49 differs fromthat shown in FIG. 40 in that a resist reflow method is adopted and nocontact holes are formed in the protective film PSV. Incidentally, FIG.50 shows a cross section taken along line 50-50 of FIG. 49, and FIG. 51shows a cross section taken along line 51-51 of FIG. 49.

<<Drain Signal Line DL>>

Cross sections of drain signal lines DL are shown in FIG. 50. Each ofthe drain signal lines DL is made of a stacked structure in which asemiconductor layer AS, a high-concentration layer d₀, a Mo layer d₁ andan ITO film ITO1 are stacked in that order. In this construction, theITO film ITO1 is formed to be extended to the side walls of the stackedstructure, that is to say, to cover the whole of the drain signal lineDL.

<<Gate Signal Line GL>>

Cross sections of gate signal lines GL are shown in FIG. 51. Each of thegate signal lines GL is formed on the transparent substrate SUB1, and ismade of a stacked structure in which the ITO film g1 and the Mo layer g2are stacked in that order. Tapered surfaces which become gradually moreopen toward the transparent substrate SUB1 are respectively formed onthe side walls of each of the gate signal lines GL.

<<Counter Voltage Signal Line CL>>

A cross section of a counter voltage signal line CL is shown in FIG. 51.The counter voltage signal line CL is made of a stacked structure inwhich the ITO film g1 and the Mo layer g2 are stacked in that order,similarly to the gate signal lines GL. The ITO film g1 is formed to bewider than the Mo layer g2 so that the ITO film g1 can serve thefunction of the counter electrode CT.

<<Drain Terminal Part DTM>>

A cross section of one of drain terminal parts DTM is shown in FIG. 52.The drain terminal part DTM is made of a stacked structure in which thesemiconductor layer AS, the high-concentration layer d₀, the Mo layer d₁and the ITO film ITO1 are stacked in that order. In this construction,the ITO film ITO1 is formed to be extended to the side walls of thestacked structure.

The drain terminal part DTM is formed by opening apertures in theprotective film PSV which covers the drain signal lines DL and exposingthe surface of the ITO film.

<<Manufacturing Method>>

FIGS. 53A to 53D are process diagrams showing one embodiment of amanufacturing method for the above-described liquid crystal displaydevice, and show a portion which includes a thin film transistor TFT anda counter electrode CT.

First of all, as shown in FIG. 53A, after a gate signal line GL, thecounter electrode CT and a counter voltage signal line CL have beenformed on the surface of the transparent substrate SUB1, an insulatingfilm GI is formed to cover the gate signal line GL, the counterelectrode CT and the counter voltage signal line CL.

A semiconductor layer AS, a high-concentration layer d₀ and a Mo layerd₁ are formed on the surface of the insulating film GI in a stackedmanner, whereby the thin film transistor TFT and a drain signal lines DLare formed by the above-described resist reflow method.

An ITO film ITO1 is formed on the entire area of the surface of thetransparent substrate SUB1 formed in this manner, and a photoresist filmPRES is formed on the surface of the ITO film ITO1 in a portion whichcorresponds to an area in which to form a pixel electrodes PX.

Then, the ITO film ITO1 is etched by using this photoresist film PRES asa mask, whereby the pixel electrodes PX is formed as shown in FIG. 53B.As shown in FIG. 53C, a protective film PSV is formed.

Then, as shown in FIG. 53D, an aperture is opened in the protective filmPSV through a photo-process (not shown), thereby preparing aconstruction in which the protective film PSV is not formed in thecentral portion of the pixel area except the periphery thereof.Incidentally, at the same time that holes are formed in this protectivefilm PSV, holes at the gate terminal parts GTM and the drain terminalparts DTM are also formed.

Incidentally, in each of the above-described embodiments, the Mo layerd₁ which constitutes the drain signal lines DL, the drain electrodes SD1and the source electrodes SD2 are formed of molybdenum (Mo), but even ifmolybdenum (Mo) is replaced with another high melting point metal suchas tungsten (W), chromium (Cr), titanium (Ti) or tantalum (Ta), it ispossible to achieve similar advantages.

In addition, an IZO (Indium-Zinc-Oxide) film may also be used instead ofeach of the above-described ITO films. As is apparent from the foregoingdescription, in accordance with the liquid crystal display deviceaccording to the invention, it is possible to obtain a construction inwhich signal lines have small wiring resistivity.

In addition, in accordance with the liquid crystal display deviceaccording to the invention, it is possible to obtain a construction inwhich few steep steps are present on the liquid-crystal-side surface ofa substrate. In addition, in accordance with the manufacturing methodfor the liquid crystal display device according to the invention, it ispossible to reduce the number of processes in the manufacturing method.

1. A liquid crystal display device comprising: substrates disposed inopposition to each other with a liquid crystal interposed there between;a thin film transistor to be driven by a scanning signal supplied from agate signal line; and a pixel electrode to be supplied with a videosignal from a drain signal line via the thin film transistor, the thinfilm transistor and the pixel being provided in each pixel area on aliquid-crystal-side surface of one of the substrates, the gate signalline being made of a multi-layered structure including: either one of anITO film and an IZO film which is formed on at least theliquid-crystal-side surface; and a layer formed to overlie the eitherone of the ITO film and the IZO film, the layer being any one of a Molayer, a W layer, a Cr layer, a Ti layer and a Ta layer, or an alloylayer made of arbitrary ones of Mo, W, Cr, Ti and Ta.
 2. A liquidcrystal display device comprising: substrates disposed in opposition toeach other with a liquid crystal interposed there between; a thin filmtransistor to be driven by a scanning signal supplied from a gate signalline; and a pixel electrode to be supplied with a video signal from adrain signal line via the thin film transistor, the thin film transistorand the pixel being provided in each pixel area on a liquid-crystal-sidesurface of one of the substrates, the gate signal line being made of amulti-layered structure including: either one of an ITO film and an IZOfilm which is formed on at least the liquid-crystal-side surface; and alayer formed to overlie the either one of the ITO film and the IZO film,the layer being any one of a Mo layer, a W layer, a Cr layer, a Ti layerand a Ta layer, or an alloy layer made of arbitrary ones of Mo, W, Cr,Ti and Ta, the pixel electrode being formed on an insulating film whichincludes as one area a gate insulating film of the thin film transistor.3. A liquid crystal display device according to claim 2, wherein thethin film transistor is made of a stacked structure in which a gateelectrode connected to the gate signal line, the gate insulating film, asemiconductor layer and a pair of electrodes formed on the upper surfaceof the semiconductor layer are stacked in that order, an extended end ofthe pixel electrode being formed on the upper surface of thesemiconductor layer to constitute one of the pair of electrodes.
 4. Amanufacturing method for a liquid crystal display device comprising thesteps of: forming, on a substrate, gate signal lines each made of astacked structure in which a transparent conductive film and a metallayer are stacked in that order; forming an insulating film to cover thegate signal lines; forming, on the insulating film, a stacked structurein which a semiconductor layer, a high-concentration layer and aconductive layer are stacked in that order; performing selective etchingof the conductive layer and the high-concentration layer by using aresist reflow method, to form drain electrodes and source electrodes forthin film transistors as well as drain signal lines and to performselective etching of the semiconductor layer; forming pixel electrodeseach of which is in part directly superposed on the source electrode ofthe corresponding one of the thin film transistors; and forming aprotective film and opening, in the protective film, apertures forexposing the respective pixel electrodes.
 5. A manufacturing method fora liquid crystal display device according to claim 4, wherein the resistreflow method includes the steps of: forming a photoresist film on anarea in which to form the drain electrode and the source electrode ofeach of the thin film transistors as well as the corresponding one ofthe drain signal lines; performing etching using the photoresist film asa mask; sagging the photoresist film to cause the photoresist film toexist at least between the drain electrode and the source electrode; andperforming etching using the sagged photoresist film as a mask.
 6. Aliquid crystal display device according to claim 4, wherein at the sametime that the pixel electrodes are formed, a transparent conductive filmis deposited on areas in which to form drain terminal parts of therespective drain signal lines.
 7. A liquid crystal display deviceaccording to claim 4, wherein at the same time that holes are opened inthe protective film, holes at gate terminal parts and drain terminalparts are opened. 8-11. (canceled)
 12. A manufacturing method for aliquid crystal display device comprising the steps of: forming, on asubstrate, gate signal lines each made of a stacked structure in which atransparent conductive film and a metal layer are stacked in that order;forming an insulating film to- cover the gate signal lines; forming, onthe insulating film, a stacked structure in which a semiconductor layer,a high-concentration layer and a conductive layer are stacked in thatorder; performing selective etching so that the conductive layer, thehigh-concentration layer and the semiconductor layer are left in each ofan area in which to form drain electrodes for thin film transistors, anarea in which to form source electrodes for the thin film transistors,an area between the drain electrode and the source electrode of each ofthe thin film transistors, and an area in which to form drain signallines; forming a transparent conductive film which constitutes pixelelectrodes; performing selective etching so that the transparentconductive film is left in each of the area in which to form the drainelectrodes for the thin film transistors, the area in which to form thesource electrodes for the thin film transistors, the area in which toform the drain signal lines, and an area in which to form the pixelelectrodes; performing selective etching of the conductive layer and thehigh-concentration layer between the drain electrode and the sourceelectrode of each of the thin film transistors by using the remainingtransparent film as a mask; and forming a protective film and opening,in the protective film, apertures for exposing the respective pixelelectrodes.
 13. A manufacturing method for a liquid crystal displaydevice according to claim 12, wherein at the same time that holes areopened in the protective film, holes at gate terminal parts and drainterminal parts are opened.
 14. A manufacturing method for a liquidcrystal display device comprising the steps of: forming, on a substrate,a transparent conductive film and a conductive layer partly superposedon the transparent conductive film, and forming counter electrodes froma single layer made of the transparent conductive film, as well as gatesignal lines and counter voltage signal lines from a stacked structurein which the transparent conductive film and the conductive layer arestacked; forming an insulating film to cover the counter electrodeelectrodes, the counter voltage signal lines and the gate signal lines;forming, on the insulating film, a stacked structure in which asemiconductor layer, a high-concentration layer and a conductive layerare stacked in that order; performing selective etching of theconductive layer and the high-concentration layer by using a resistreflow method, to form drain electrodes and source electrodes for thinfilm transistors as well as drain signal lines and to perform selectiveetching of the semiconductor layer; forming pixel electrodes each madeof a transparent conductive film which is in part directly superposed onthe source electrode of the corresponding one of the thin filmtransistors; and forming a protective film and opening, in theprotective film, apertures for exposing the respective pixel electrodes.15. A manufacturing method for a liquid crystal display device accordingto claim 14, wherein the resist reflow method includes the steps of:forming a photoresist film on an area in which to form the drainelectrode and the source electrode of each of the thin film transistorsas well as the corresponding one of the drain signal lines; performingetching using the photoresist film as a mask; sagging the photoresistfilm to cause the photoresist film to exist at least between the drainelectrode and the source electrode; and performing etching using thesagged photoresist film as a mask.
 16. A liquid crystal display deviceaccording to claim 14, wherein at the same time that the pixelelectrodes are formed, a transparent conductive film is deposited onareas in which to form drain terminal parts of the respective drainsignal lines.
 17. A liquid crystal display device according to claim 14,wherein at the same time that holes are opened in the protective film,holes at gate terminal parts and drain terminal parts are opened. 18-21.(canceled)
 22. A manufacturing method for a liquid crystal displaydevice comprising the steps of: forming on a substrate a stackedstructure in which a first material layer and a second material layerare stacked in that order; forming a photoresist film on the uppersurface of the stacked structure and forming a photoresist film oflarger film thickness on a first area as well as a photoresist film ofsmaller film thickness on a second area by using half exposure; etchingthe second material layer by using the photoresist film as a mask andvanishing the photoresist film of smaller film thickness; etching thefirst material layer by using the second material layer as a mask; andforming a stacked structure in which the first material layer and thesecond material layer are stacked in that order on the first area of thesubstrate, and forming the first material layer on the second area. 23.A manufacturing method for a liquid crystal display device whichincludes pixel areas provided on a liquid-crystal-side surface of eitherone of substrates disposed in opposition to each other with a liquidcrystal interposed there between, each of the pixel areas beingsurrounded by gate signal lines disposed to be extended in the xdirection and to be juxtaposed in the y direction and drain signal linesdisposed to be extended in the y direction and to be juxtaposed in the xdirection; each of the pixel areas including: a switching element to bedriven by a gate signal from either one of the gate signal lines; apixel electrode to be supplied with a video signal from a drain signalline via the switching element; and a counter electrode for causing anelectric field to be generated between the counter electrode and thepixel electrode, the manufacturing method comprising the steps of:forming, on either one of the substrates, a stacked structure in which afirst material layer and a second material layer are stacked in thatorder; forming a photoresist film on the upper surface of the stackedstructure and forming, by using half exposure, a photoresist film oflarger film thickness on each area in which to form a gate signal lineas well as a photoresist film of smaller film thickness on each area inwhich to form a counter electrode; etching the second material layer byusing the photoresist film as a mask and vanishing the photoresist filmof smaller film thickness; and etching the first material layer by usingthe second material layer as a mask.
 24. A manufacturing method for aliquid crystal display device according to claim 21, wherein the firstmaterial layer is made of an ITO film, and the second material layer ismade of a Mo layer